Theoretical Investigations of Structural Properties and Isomerization mechanism of Weakly Bound Silicon Monocation Binary Carbonyl and Isocarbonyl Complexes

摘要

The structures, properties, and bonding character for the Si binary carbonyl and isocarbonyl monovalent cations, Si(CO)_2~+ (~2B_1), COSiCO~+ (~2A"), and Si(OC)_2~+ (~2B_1), in their doublet states have been investigated using four density functional theory (DFT) methods and the MP2 method with the 6-311-G~* and aug-cc-pvtz basis sets. Results indicate that, of the binary carbonyl and isocarbonyl Si cations, there exist three stable isomers; all of them exhibit V-type structures in the doublet state, and no linear geometry has been found. The most stable isomer is Si(CO)_2~+ (~2B_1), the dicarbonyl Si cation, and can be assigned to the global energy minimum. The other two isomers are COSiCO~+ (~2A"), the carbonyl and isocarbonyl Si cation, and Si(OC)_2~+ (~2B_1), the di-isocarbonyl Si cation. They lie 22.8 and 40.9 kcal/mol above Si(CO)_2~+ (~2B_1) at the CCSD(T)/aug-cc-pvtz level. The calculated results and bonding analysis have indicated that the binding strength of CO with Si~+ by the C-terminal is stronger than that by the O-terminal. but the C-O bond weakening caused by the O-terminal interaction with Si~+ is slightly greater than that by the C-terminal. The corresponding coordination bond length to the Si~+ center by the O-terminal is significantly larger than that by the C-terminal. The vibrational frequencies and the charge populations also support the above analysis. These binary carbonyl or isocarbonyl Si complex cations have essentially silene cation character and should be referred to as binary carbonyl silene. Further comparison of the CO-binding energies among three binary carbonyl Si complex cations indicates that the second CO-binding energy (26.2 kcal/mol) of Si(CO)_2~+ (~2B_1) is greater by ～5 kcal/mol than the first one (21.5 kcal/mol) at the CCSD(T, full)/6-311 + G~*/MP2(full)/6-311+G~* level, but it is the inverse in the Si(OC)_2~+ (~2B_1) species. This observation, together with the bonding analysis, has implied that the possibility is small for the existence of the polycarbonyl Si cations with more than three CO's. To explore the state-state correlations among three isomers, searches of the transition states and the isomerization mechanism have also been performed. Results indicate that there are two single-ring transition states, which correlated with Si(CO)_2~+ (~2B_1) and (CO)Si(CO)~+ (~2A") (TS1), and (CO)Si(CO)~+ (~2A") and Si(OC)_2~+ (~2B_1) (TS2), respectively. The forward (inverse) activation energies are 29.6 kcal/mol (12/0 kcal/mol) for TS1 and 20.0 kcal/mol (6.9 kcal/mol) for TS2, respectively. No direct correlation is found for Si(CO)_2~+ (~2B_1) and Si(OC)_2~+ (~2B_1). The isomerizations for this kind of weak interaction system generally adopt the loosening-rotating mechanism.